update php-coveralls entry

Author: adriangibbons

composer.json

@@ -7,7 +7,7 @@
     "require-dev": {
         "phpunit/phpunit": "4.8.*",
         "squizlabs/php_codesniffer": "2.*",
-        "satooshi/php-coveralls": "dev-master"
+        "satooshi/php-coveralls": "^2.0"
     },
     "autoload": {
         "psr-4": {